STAT3-deficient mouse embryos cannot develop beyond embryonic day 7, when gastrulation begins.[9] It appears that at these early stages of development, STAT3 activation is required for self-renewal of embryonic stem cells (ESCs). Indeed, LIF, which is supplied to murine ESC cultures to maintain their undifferentiated state, can be omitted if STAT3 is activated through some other means.[10]

STAT3 is essential for the differentiation of the TH17 helper T cells, which have been implicated in a variety of autoimmune diseases.[11]

Loss-of-function mutations in the STAT3 gene result in Hyperimmunoglobulin E syndrome, associated with recurrent infections as well as disordered bone and tooth development.[12]

Gain-of-function mutations in the STAT3 gene have been reported to cause multi-organ early onset auto-immune diseases; such as thyroid disease, diabetes, intestinal inflammation, and low blood counts,[13] while constitutive STAT3 activation is associated with various human cancers and commonly suggests poor prognosis.[14][15][16][17] It has anti-apoptotic as well as proliferative effects.[14] STAT3 can promote oncogenesis by being constitutively active through various pathways as mentioned elsewhere. Very recently a tumor suppressor role of STAT3 has also been reported.[18][19][20] In the report on human glioblastoma tumor, or brain cancer, STAT3 was shown to have an oncogenic or a tumor suppressor role depending upon the mutational background of the tumor. A direct connection between the PTEN-Akt-FOXO axis (suppressive) and the leukemia inhibitory factor receptor beta (LIFRbeta)-STAT3 signaling pathway (oncogenic) was shown. In addition, two recent studies performed in APC mutant mice showed that STAT3 has an inhibiting role in colon carcinogenesis depending on tumor stage.